There are various tools used in the oil and gas exploration and production industry featuring extendable cutters, including under-reamers. The cutters may be actuated by the application of weight, or by fluid pressure. Examples of such tools are described in the applicant's International Patent Application Nos. WO 00/31371 and WO 2004/097163, the disclosures of which are incorporated herein by reference.
An under-reamer will typically be incorporated in a drill string above the drill bit, and the cutting blades of the under-reamer, or a blade-extending arrangement, will initially be restrained in a retracted position, typically by shear pins or the like. This allows the operator to use the drill bit to drill through the cement plug and the shoe at the lower end of the last section of casing with the under-reamer located within the casing. Only when the hole has been drilled to the extent that the under-reamer is located beyond the end of the casing is the under-reamer activated, and the cutters extended, to ream the hole cut by the drill bit to a diameter larger than the existing casing.